This invention relates generally to automatic frequency control circuits and specifically to automatic frequency control circuits for use with television signals having offset picture carrier frequencies.
In U.S. Pat. No. 3,796,824, issued Mar. 12, 1974 to R.F. Baker and assigned to Zenith Radio Corporation, an automatic frequency control circuit for a television receiver is described which utilizes a discriminator and achieves extended pull-in by use of a so called "negative hook" in the vicinity of the sound intermediate frequency (IF) carrier. As discussed in the patent, with varactor tuners particularly which are sensitive to even small D.C. changes in discrimination output, the "negative hook" extends the AFC pull-in range especially under poor signal or tuning conditions. The separate control effects from the picture IF carrier and the sound IF carrier tend to augment each other in contrast with conventional AFC systems characteristics. Thus the pull-in range is enhanced.
U.S. Pat. No. 4,091,421, issued May 23, 1978 to Michael E. Long, and also assigned to Zenith Radio Corporation, discloses a balanced multiplier product detector, especially for use with linear synchronous detectors having no need for sound trapping, which has a large "negative hook" and a small positive response area on the low frequency side to achieve enhanced pull in by using the sound carrier to produce complementary control effects for both high and low signal offsets. A drawback of both circuits is that two tuning adjustments are required which of course adds to labor costs.
The above described patented AFC circuits with negative hooks work very well with assigned frequency broadcast signals which generally have a sound carrier that is 7 to 10 dB down from the picture carrier.
The advent of cable systems however has given rise to television signals that are not subject to the stringent FCC regulations governing over-the-air transmissions. On such cable systems, the sound carrier may be as much as 20 dB down from the picture carrier and additionally, the picture carrier may be offset by 1 to 2 MHz. In an HRC (harmonically related carrier) mode most channels are down-shifted 1.25 MHz whereas in an IRC (inter-harmonically related carrier) mode channels 5 and 6 are down-shifted 2 MHz. These frequency shifts and sound reductions help the cable operators reduce radio frequency interference problems to a significant degree. Unfortunately, the effect on the television receiver is not beneficial. Specifically the AFC circuit in the receiver must be capable of extended pull-in to assure proper "locking" to the signal having an offset carrier frequency and a very low level sound carrier.
If for example the picture carrier is offset to produce an IF frequency of 47 MHz (rather than 45.75 MHz) it will be nearly outside of the conventional AFC response characteristic and therefore produce very little control effect for pull-in. With the response characteristic discussed, adjacent signal carriers may fall in the positive hump adjacent to the negative hook and yield an erroneous control effect. A modified circuit without the positive hump removed this problem but at the expense of also diminishing the amplitude of the negative hook portion. This low level sound carriers in the negative hook portion of the response characteristic had a diminished control effect. Thus, the result is often insufficient control voltage for proper AFC action, or erroneous AFC action. Further, in the circuit with the diminished negative hook, the offset color subcarrier could occur at a point between the negative hook portion and the higher frequency positive going portion of the AFC response which could cause the upper sidebands of the color signal to have more control effect than the lower sidebands. Thus the AFC action could produce a correction signal tending to tune the receiver in the wrong direction.
In a practical receiver installation with direct channel number tuning, a tuner phase-lock scan system is used to tune the receiver's local oscillator frequency to the incoming signal frequency. If, for example, channel 2 is entered, the varactor tuner is initially tuned to the assigned frequency of channel 2. The AFC circuit produces an output causing the receiver tuning system to scan in the indicated direction toward the appropriate limit frequency for the channel and then "come around" to scan toward the middle of the band from the other direction. If the receiver AFC doesn't lock in, scanning stops at the assigned frequency. If the AFC response characteristic is such that pull-in doesn't occur, the tuning system is locked out and the channel number must be reentered. Since the band is only scanned once, scanning is relatively slow to avoid missing weak signals. Thus the time between the entry of the channel and lock-in of the receiver tuner is undesirably extended should scanning be started in the wrong direction because of an erroneous initial AFC response.
Thus there is a need in the art for an AFC system for a television receiver which is capable of unambiguously producing AFC correction signals in the presence of television signals that are substantially offset from assigned frequencies. Preferably such a system should include a minimum of tuned circuits for ease in alignment and low cost.